The present invention refers to steam or hot water boilers of the type including a convection part, formed as a cylindrical drum having plane end plates, within which at least two groups of smoke tubes are mounted concentrically with respect to a flame tube and extending between the end plates of the drum, an annular turning chamber, providing passage for the combustion products from one group of smoke tubes to the other group, being arranged adjacent to one of said end plates, being outwardly defined by a wall being parallel thereto, and where further an oil burner is fitted in the outward end of the flame tube. The aim of the invention is to provide simple and expedient means for preheating the combustion air, while simultaneously taking care of residual heat in the combustion gases.